function [vx, vF] = fn_combine(vk, P, Q, vu, u_dH)

[tvk2, tv2] = fn_prod(vk, Q, vu, u_dH);
vk2 = [tvk2, vk];
vx = unique(sort(vk2));
vF = interp1(vk, P, vx, 'pchip') ...
    - interp1(tvk2, tv2, vx, 'pchip');

end
